US 7049014 B1
A fuel cell using a secondary alcohol such as 2-propanol as fuel is disclosed. The fuel is oxidized directly at the anode without any reforming. Such a direct secondary alcohol fuel cell (D2AFC) possesses a much higher performance than a direct methanol fuel cell, especially at current densities less than 200 mA/cm2. In addition, fuel loss due to crossover in a direct 2-propanol fuel cell (D2PFC) is less than one-sixth of that in a direct methanol fuel cell (DMFC).
1. A direct oxidation electrochemical fuel cell comprising a stack of electrode and membrane elements, said direct oxidation electrochemical fuel cell producing electrical current through the oxidation of a fuel selected from the group: butanone (CH3CH2COCH3), and pentanone (CH3COCH2CH2CH3).
2. The electrochemical fuel cell according to
3. The solid membrane fuel cell according to
4. The solid membrane fuel cell according to
5. The solid membrane fuel cell according to
6. A direct oxidation electrochemical fuel cell comprising a stack of electrode and membrane elements, said direct oxidation electrochemical fuel cell producing electrical current through the oxidation of a secondary alcohol comprising glyceraldehyde (CH2OHCHOHCOH).
7. The electrochemical fuel cell according to
8. The solid membrane fuel cell according to
9. The solid membrane fuel cell according to
10. The solid membrane fuel cell according to
This invention relates to direct oxidation fuel cells, and particularly to a secondary alcohol fuel cell used in a direct oxidation fuel cell using 2-propanol as its fuel.
Hydrogen is the cleanest and most efficient fuel used in fuel cells. It is widely used in low temperature fuel cells like proton exchange membrane (PEM) fuel cells, alkaline fuel cells and phosphoric acid fuel cells, because its oxidation rate at the anode is high, even at room temperature. However, producing pure hydrogen is not a trivial task. Hydrogen is normally produced through reforming hydrocarbon fuels, such as methane, propane, and methanol. This not only makes the entire fuel cell system more complicated, it also dramatically increases the cost. Moreover, any carbon monoxide (CO) remaining in the reformed gas, even at ppm levels, will poison the electrodes of a PEM fuel cell and reduce its performance. In addition, transporting and storing hydrogen is very difficult, presenting a safety hazard.
The problems associated with hydrogen have encouraged scientists to look for other fuels that can be directly oxidized without requiring a reforming step. Methanol, the simplest alcohol containing only one carbon atom, is the most popular and widely used fuel in this regard. A direct oxidation fuel cell using methanol as the fuel is called a direct methanol fuel cell (DMFC).
In U.S. Pat. Nos. 3,013,908 and 3,113,049, a DMFC is described. Liquid feed direct methanol fuel cells have been in use from the early 1960s. These early DMFCs used liquid electrolyte like a dilute sulfuric acid for proton transportation. Major problems were encountered using the sulfuric acid electrolyte, such as corrosion of cell materials, poisoning of the electrodes by the adsorption of sulfate anions, and leakage of the electrolyte through the surrounding materials. For example, the electrolyte could gradually leak out through the pores of the air cathode, also causing fuel loss and cathode poisoning.
In order to alleviate leakage, a solid proton exchange membrane was interposed between the anode and cathode. Nafion® perfluorinated polymer, made by E. I. DuPont, was used in U.S. Pat. Nos. 4,262,063 and 4,390,603.
U.S. Pat. No. 4,478,917 used a membrane comprising styrene-divinylbenzene co-polymers with sulphonate groups.
In recent years, the use of liquid electrolyte has not been frequent in a DMFC. U.S. Pat. No. 5,599,638 was granted to Surampudi et al for just using a proton exchange membrane like Nafion as the electrolyte. Nafion membranes have excellent chemical, mechanical, thermal, and electrochemical stability and their ionic conductivity can reach as high as 0.1 S/cm. The kinetics of methanol oxidation and oxygen reduction at the electrode/membrane/electrode interfaces has been found to be more facile than at the electrode/sulfuric acid/electrode interfaces. Corrosion of cell materials becomes less severe since the fuel and water solution is free from sulfuric acid. The Nafion cell can be operated at temperatures as high as 120° C., while a sulfuric acid cell tends to degrade at temperatures higher than 80° C. Also, the absence of conducting ions in the fuel and water solution substantially eliminates the parasitic shunt currents in a multi-cell stack. Such a fuel cell is illustrated in U.S. Pat. No. 6,248,460, a continuation of U.S. Pat. No. 5,599,638, granted to Surampudi et al.
In U.S. Pat. No. 5,904,740, granted to Davis, a fuel cell with formic acid added into the methanol and water solution for the conduction of protons within the anode structure is shown. The formic acid is claimed to improve ionic conductivity and to be a clean burning fuel that does not poison the catalysts.
Unfortunately, methanol poses the serious problem of penetrating and crossing through Nafion membranes as well as other types of proton exchange membranes, via physical diffusion and electro-osmotic proton drag. Such crossover not only results in a large waste of fuel, it also greatly lowers cathode performance. Most of the methanol crossover will be electrochemically and chemically oxidized at the cathode. These oxidation reactions not only lower the cathode potential, they also consume some oxygen. Should the reaction intermediate comprise carbon monoxide, it can be adsorbed onto the catalyst surface, thus poisoning the cathode. This will further lower the performance of the fuel cell.
In U.S. Pat. No. 5,672,438, a thin layer of polymer having a higher ratio of backbone carbon atoms to those of the cationic exchange side chain is illustrated. This polymer reduces the methanol crossover rate, although the membrane resistance increases. It was suggested that the polymer with higher carbon atom ratios be preferably orientated on the anode side. Prakash et al described a polymer membrane composed of polystyrene sulfonic acid (PSSA) and poly(vinylidene fluoride) (PVDF), in WO 98/22989. The PSSA-PVDF membrane exhibited lower methanol crossover, translating into higher fuel and fuel cell efficiencies. Pickup et al suggested a modified ion exchange membrane possessing lower methanol crossover, in WO 01/93361A2. Existing membranes comprising Nafion were modified in situ by polymerizing monomers, such as aryls, heteroaryls, substituted aryls, substituted heteroaryls, or combinations thereof. The modified membrane exhibited reduced permeability to methanol crossover, often without a significant increase in ionic resistance.
Another barrier to the commercialization of DMFCs is the sluggishness of the methanol oxidation reaction. Moreover, some intermediates from methanol oxidation, like carbon monoxide, can strongly adsorb to the surface of catalysts, poisoning them, as aforementioned. Platinum alloys such as Pt/Ru have a much higher CO-tolerance, so they are widely used as the anode catalyst. Other short chain organic chemicals like formic acid, formaldehyde, ethanol, 1-propanol, 1-butanol, dimethoxymethane, trimethoxymethane, and trioxane have been suggested as fuels in direct oxidation fuel cells. U.S. Pat. No. 5,599,638 describes experimental results of using dimethoxymethane, trimethoxymethane, and trioxane in fuel cells. It was claimed that dimethoxymethane, trimethoxymethane, and trioxane could be oxidized at lower potentials than methanol, and thus would be better fuels than methanol. It was also claimed that only methanol was found to be the intermediate product from the oxidation of these fuels, thus there was no concern (i.e., methanol is ultimately oxidized to carbon dioxide and water). Using Nafion 117® as the membrane and oxygen as the oxidant, with a pressure of 20 psig, cell voltages of 0.25 V, 0.50 V, and 0.33 V were achieved at a current density of 50 mA/cm2, when dimethoxymethane, trimethoxymethane, and trioxane were used at cell temperatures of 37° C., 65° C., and 60° C., respectively. However, these performances are very low and would not provide a commercial fuel cell.
The present invention provides a direct oxidation fuel cell performing much better than DMFCs, using secondary alcohol as the fuel.
It is an object of this invention to provide a fuel cell using secondary alcohols as the fuel.
It is another object of the invention to provide a fuel cell using 2-propanol as the fuel.
It is yet another object of this invention to provide a fuel cell whose fuel crossover is much less than a DMFC using methanol.
A secondary alcohol, isopropanol (2-propanol), is used as fuel in a direct oxidation fuel cell. Such a fuel cell shows much higher performance than a direct methanol fuel cell and other currently reported direct oxidation fuel cells. The isopropanol crossing through a membrane is less than one-sixth that of methanol, so a direct 2-propanol fuel cell (D2 PFC) can have much higher fuel and fuel cell efficiencies. Other secondary alcohols that could be used are glycerols, propylene glycol (CH3CHOHCH2OH), propylene glycol (CH3CHOHCH2OH), glyceraldehyde (CH2OHCHOHCOH), ethylene glycol (CH2OHCH2OH), short chain alkanones such as propanone (CH3COCH3), butanone (CH3CH2COCH3), and pentanone (CH3COCH2CH2CH3, CH3CH2COCH2CH3). The direct oxidation electrochemical fuel cell is selected from a group of fuel cells consisting of a liquid electrolyte fuel cell, a solid membrane fuel cell, an alkaline fuel cell, a phosphoric acid fuel cell, a molten carbonate fuel cell, and a solid oxide fuel cell. The membrane includes a membrane element selected from a group consisting of non-fluorinated, partially fluorinated, and perfluorinated membranes.
A complete understanding of the present invention may be obtained by reference to the accompanying drawings when considered in conjunction with the subsequent detailed description, in which:
Generally speaking, the invention features a fuel cell using a secondary alcohol as fuel. Isopropanol (2-propanol) was used as fuel in a direct oxidation fuel cell resulting in much higher performance than a direct oxidation methanol fuel cell.
Test Apparatus and Experiments
Tests were conducted using a 25 cm2 fuel cell test fixture. Pt/Ru and Pt blacks were used as the anode and cathode catalysts and were applied to plain and Teflon®-treated 9-mil Toray paper, respectively to achieve a Pt/Ru and Pt loading of 4.8 mg/cm2, respectively. These electrodes were hot-pressed onto Nafion 112 membrane to form a membrane-electrode assembly.
Alcohols were mixed with water before they were pumped into the cell by a micro-pump. The mixture was then re-circulated back to the mixing tank. The flow rates were controlled by a DC power supply. A condenser was used to condense the alcohol in the vapor phase and to allow the release of gaseous CO2. The temperature of the mixing tank was controlled by a hot plate. The connection between the mixing tank and the cell was heated by heating tapes when needed. The temperatures of the mixing tank, alcohol mixture inlet and outlet, and air outlet were monitored by thermocouples. Air rather than oxygen was supplied to the cell by a compressor, and its flow rate was adjusted using a flow meter. Air was used at ambient temperature and pressure.
Now referring to
The highest current density of 250 mA/cm2 was achieved when a 1.0 M 2-propanol solution was used, despite a lower cell temperature of 40° C., as shown in
A highest current density of between 250 and 320 mA/cm2 was achieved when the fuel cell temperature was increased from 40° C. to 60° C., as shown in
The D2 PFC showed slightly better performance at an air flow rate of 920 (curve 42) than at an air flow rate of 180 ml/min (curve 43), while for the DMFC, an airflow rate of 920 ml/min (curve 40) gave a much higher performance than an airflow rate of 180 ml/min (curve 41). The performance of the DMFC at an airflow rate of 180 ml/min was extremely low (curve 41).
At the same 2-propanol concentration of 1.0 M, the OCV increased by 10 mV as the cell temperature increased from 40° C. (curve 48) to 60° C. (curve 49), but the increase was less than 4 mV when the cell temperature was further increased from 60° C. (curve 49) to 80° C. (curve 50).
At the same cell temperature of 60° C. the OCV decreased slightly as the 2-propanol concentration was increased from 0.5 M (curve 52) to 1.0 M (curve 49). However, a much larger decrease was observed when the concentration was further increased from 1.0 M (curve 49) to 2.0 M (curve 5L).
At an airflow rate of 180 ml/min. the cell actually had a slightly lower OCV at 80° C. than at 60° C.
Higher alcohol concentration gave a smaller OCV. For the DMFC using 1.0 M methanol solution, its OCV dropped significantly from 0.57 V to 0.47 V when the airflow rate declined from 397 to 180 ml/min (curve 54).
Increasing the methanol concentration from 0.5 M (curve 53) to 1.0 M (curve 54) decreased the OCV of the DMFC as much as 50 mV. In contrast, when 2-propanol concentration was increased from 0.5 M (curve 55) to 1.0 M (curve 56), the OCV of the D2 PFC only declined slightly.
As 2-propanol crosses through the membrane to reach the cathode it is oxidized by the applied voltage. The value of the measured current density represents how fast 2-propanol crosses through the membrane. All the curves from 57 through 61 showed four distinct regions: From 0.0 to 0.2 V, the crossover current increased quickly with the applied voltage; from 0.2 to 0.5 V the crossover current stayed flat. From 0.5 to 0.8 V the crossover current increased quickly again. From 0.8 V to 0.95 V the crossover current approached a plateau. The flat region from 0.2 to 0.5 V was found to be due to quick poisoning of the cathode by the oxidation intermediates of 2-propanol. Each data point would go lower if a longer time was observed before the data was recorded.
Platinum was used as the catalyst for the cathode. As the applied voltage went higher than 0.5 V, the catalyst surface seemed to be cleaned by the positive voltage and the currents increased until a mass transport limitation of 2-propanol through the membrane was approached at voltages higher than 0.8 V, and preferably, at a voltage higher than 0.9 V. Therefore, a more accurate crossover current is measured at voltages higher than 0.8 V.
At the same 2-propanol concentration of 1.0 M, the crossover current increased almost linearly as the cell temperature was increased from 40° C. (curve 57) to 60° C. (curve 58) and then to 80° C. (curve 59). At the same cell temperature of 60° C. the crossover current increased significantly from 0.5 M 2-propanol (curve 61) to 1.0 M (curve 58) and then to 2.0 M (curve 60).
At the same cell temperature of 60° C., the crossover current increased significantly from 0.5 M 2-propanol (curve 65) to 1.0 M (curve 63), and then to 2.0 M (curve 66).
Some of the above experimental results are summarized in Table 1 and Table 2, below.
Equations (1) and (2) show the oxidation reactions of methanol and 2-propanol, assuming complete reactions to form CO2 as the final product:
For each methanol molecule, 6 electrons are produced, while for each 2-propanol molecule, 18 electrons are produced. In other words, for a complete oxidation of each 2-propanol molecule, three times as large a current should be observed compared to a complete oxidation of each methanol molecule. Therefore, the amount of 2-propanol crossing through the membrane is less than one-sixth that of methanol based on the crossover currents shown in
Another advantage of 2-propanol over methanol is its higher electrochemical energy density. 2-propanol has a similar density as methanol (0.785 vs. 0.791 g/cm3) and the molecular mass of 2-propanol (60.10 g/mol) is less than double that of methanol (32.04 g/mol). The complete oxidation of one 2-propanol molecule produces three times as much electrons as one methanol molecule and the electrochemical energy density of 2-propanol is more than 1.5 times that of methanol, at per unit volume or mass.
Still another advantage of 2-propanol over methanol is its lower toxicity. Handling 2-propanol is much safer than handling methanol. Furthermore, 2-propanol gives off a strong smell, so any leakage of 2-propanol can be detected immediately.
Still another advantage of 2-propanol over methanol is its lower activation voltage. Based on the data in
Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.
Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.